"UN/NA numbers (the four digit number) found on bulk placards refer to specific chemicals or groups of chemicals and are assigned by the United Nations and/or the United States Department of Transportation." from http://environmentalchemistry.com/yogi/hazmat/placards/
Link also has a way to look up the numbers to find out what the cargo is. List is way too long to get into here.
self heating or pyrophoric materials
It is a DOT hazardous materials placard indicating compressed oxygen.
The placard for propane is Class 2.1. It is a diamond-shaped placard with a red upper half and a lower half that is predominantly white, displaying the Class number and the UN identification number for propane.
Explosives
This is the UN Identification Number.
The word placard (noun meaning a sign, a notice, an advertisement) has no direct antonym; the closest would be 'without placard' or 'placard-less'.
Someone has misspelled my name on this placard. A sandwich sign is a two-faced placard!
In French, "placard" is masculine.
symbol, UN hazard class number, color, UN identification number
A poisonous gas placard typically features the hazard symbol for toxic chemicals, which is a skull and crossbones symbol. The placard may also include the UN number for the specific poisonous gas, as well as any additional information required by regulations. The placard is usually in a diamond shape and is orange with black symbols.
The number 4 on the NFPA 704 placard indicates the highest hazard in any of the three categories (health, fire, reactivity).
the placard have been posted everywhere.